In oil recovery and storage operations, hydrocarbon gases are produced from oil condensate storage tanks through flash losses, working losses, standing losses, and breathing losses, where liquid hydrocarbons evaporate into gaseous form. Working losses occur when the liquids are agitated, i.e., when new liquids are pumped into the tanks; breathing losses are primarily the result of diurnal heating of the tanks; and flash losses occur as the result of a sudden pressure drop which occurs when liquid hydrocarbons move from a separator that operates at elevated pressures (i.e. approximately 30-500+ psig) to an oil condensate storage tank at much lower pressure (i.e., 0 to 1 psig). Flash losses account for a significant portion of total losses. Collectively, these losses of hydrocarbon gases are referred to herein interchangeably as “flash emissions,” “vent gas,” “flash gas,” “vapor,” “emissions” and combinations thereof.
Vapor emissions include Volatile Organic Compounds (VOC) and therefore pose a hazard to air quality as they form ground level ozone when they react with NOx. Historically, such vent gases were vented through relief valves of the storage tanks into the atmosphere.
To protect the environment and the health of the public at large, the permissible levels for ground level ozone and, in turn, uncontrolled flash emissions from oil storage tanks are regulated by various regulatory agencies at federal and state levels. Methods of flash emissions control include flaring, where the flash emissions are burned off, and more advanced systems of vapor recovery that capture, compress, and inject these flash emissions into nearby gas gathering pipelines, such as described in U.S. Pat. No. 7,350,581.
Vapor recovery systems have been developed that capture flash emissions using flexible storage tanks (also known as “bladders”) at or near the front end of such systems, such as described in co-pending U.S. patent application Ser. No. 13/365,247 filed by the present inventors, and such bags/flexible storage tanks accommodate for surges in the vapor emissions. These flexible storage tanks in the vapor recovery systems can be useful in applications where the associated flash emissions may be produced intermittently, such as with vertical or directional wells.
However, use of flexible storage tanks/bags in vapor recovery systems is primarily utilized to mitigate flash gas surges associated with intermittent production methods (i.e. plunger lift well operation) and can increase the size/footprint of such vapor recovery systems, which may be inappropriate for some situations.